"We will then see whether further action is required later in the week," he said.

He said the staff were disappointed by the pay offer after previously receiving a letter from the company's chief executive, Mark Irwin, which praised their work.

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"Since joining Serco in January I have been incredibly impressed by the staff I have met and your commitment to making a positive difference," Mr Irwin wrote.

"So as we come together this week to celebrate our achievements from the past 25 years, I want to take a moment to thank you for your contribution, and to extend that gratitude to partners and families who support you in what you do."

Mr Clifford said staff believed Serco's wage offer did not keep up with the cost of living.

"CPI nationally is running at 2.5 per cent, so it doesn't even keep them up with CPI," he said.

"The company is offering 2.2 per cent, while the average pay increase in enterprise bargaining agreements according to the federal government is 3.8 per cent," he said.

Serco was contacted for comment on Friday afternoon, but had not replied by Friday evening.